Depends on the gas mix and the application. The gas mixes used at depth in technical diving are usually hypoxic, since oxygen toxicity becomes an issue with ordinary air at an ambient pressure of about six and a half bar or depths of around fifty meters; heliox, for example, is usually around ten percent oxygen. On the other hand, it's fairly common for the gas mixes used during the decompression phase of a technical dive to be richer in oxygen than air is, since that helps flush nitrogen out of your tissues.

This isn't usually an issue for recreational divers, though, who generally don't dive below forty meters and use pressurized air or, more rarely, enriched nitrox mixtures. At those shallower depths, you compensate for the richer breathing gas by breathing in a slower, more controlled fashion than you would on the surface, though this has more to do with conserving gas and controlling buoyancy than it does with oxygen issues.